Most powerful Elf on Middle Earth (3rd age)

[Keeper of Dol Guldur]

Elrond, or Galdriel. Elrond, or Galadriel? Who is more powerful? Well, in a spiritual way Galadriel is certainly, and when it comes to 'magic' (magic to the Hobbits and men, anyway, as it could be mistaken for an insult) sorts of things (like the mirror, or the cloaks). But Elrond is of the line of Earendel, the joined houses of Beren and Luthien and Tuor and Idril, also of Turgon lord of Gondolin and Thingol of Doriath, and Melian the Maia. Galadriel was certainly the older and more experienced, but Elrond had the knowledge of many new things, and was closer to men or other races. Also, he knew better the war tactics, likely, and had the experience of prior contact to the Ring, though he never of course touched it. So I'd say they're even.

As for the most powerful elf in the third age, it was Celeborn. Greatest of Elves, he was even referred to as. As for the greatest 3rd age elf warrior, Glorfindel. The greatest mariner, Cirdan.